Rendered image is just grey
You have the focus of the camera set to 0  and are using a lens aperture of 1.0. Basically your scene is so out of focus that you only see gray. Change the distance value to the actual distance form the camera to your object, and maybe try a higher number for the f-stop.

To find where your camera is focusing you can enable display->Limits. If you move the focus you'll see a cross mark that indicates where the focus point is:



A trick you can do is use an empty. Move the empty to the place you want the camera to focus on and select the empty (or any other object as the focus target).